This invention relates to the transfer of rotary to linear motion and in particular to mechanisms for pre-loading a drive rod with a free turning roller mechanism.
Use of a linear actuator to drive structures such as satellite tracking antennas are commonly known. Typical linear actuators employ a threaded rod and a matching threaded nut assembly for translating rotational motion of the rod into linear motion of apparatus attached to the nut assembly. By placement of the nut assembly at a moment arm with respect to a pivotally mounted antenna structure, the linear translation of the nut assembly can be employed to rotate antenna structures in either elevation or azimuth.
Unfortunately, typical threaded rod assemblies of the known designs are subject to backlash and wear which limit the directional accuracy of antenna structures. In addition, a threaded rod assembly typically does not have overload protection because of its inherent inability to slip under heavy linear loads. Consequently, other types of linear actuators are finding more and more favor in the specialized fields such as antenna rotation.
For example, a threadless screw principle is employed for high precision linear actuation without backlash. A threadless linear actuator comprises a free-turning roller mounted to engage a round shaft. The roller is typically formed of a straight right circular cylinder having a central axis at an angle relative to the central axis of the driven rod. As the rod is rotated about its central axis, the cylindrical roller translates the rotary motion of the rod into a linear motion of the roller assembly along the axis of the rod.
Pre-loading of the roller assembly against the rod has been found to be a problem. It is necessary to pre-load the roller assembly against the rod to prevent slippage. Too much pre-load may create undesired binding between the rod and the roller assembly. However, requirements for pre-load typically vary with position of the roller assembly along the rod.
Various solutions to the pre-loading problem have been described in connection with the threadless screw principle in the patent literature. For example, U.S. Pat. No. 2,204,638 describes an early door-opening mechanism in which free-rolling cylinders exert traction force on a rotating shaft for opening and closing a garage door structure. Traction force is manually adjustable by the tightening of a spring-loaded mechanism to urge pivotally mounted rollers into contact with the driving rod. There is however no suggestion of how the traction force could be rendered adjustable during the operational stroke of the mechanism. The contact area between the rotating cylinders and the driven rod is conventional. Namely, the contact area is a straight, short line at an angle to the central axis of the driven rod. The length of the line is dependent upon the relative compliance of the cylinder and the driven rod. The contact load is applied only by the forceful application of the cylinders radially against the rod.
U.S. Pat. No. 2,382,105 describes a roto-thrust converter in which rollers are also cylindrical and in which pre-loading is adjustable but only for fixed, stable conditions. Pre-loading is provided at each individual cylindrical roller, as each roller is mounted on a plunger attached to a linear adjustment mechanism.
U.S. Pat. No. 3,272,021 describes a linear actuator for use in opening and closing a vehicle window in which a driven rod rotates with respect to skew-mounted, cylindrical rollers. Of particular interest in this patent is the fact that the driven rod is of non-uniform diameter such that the position of the skew-mounted rollers on the rod determines the pre-load between the rod and the abutting rollers.
U.S. Pat. No. 2,619,346 appears to the applicant to be most relevant to the present invention of the art known to the applicant. In U.S. Pat. No. 2,619,346 a set of concave rollers is shown with a spring pre-loading mechanism that forces one end of each roller set into engagement with a rotating drive rod. Pre-load force can be varied by changing compression forces of springs acting on each respective roller thereby to force the roller into tighter radial engagement with the drive rod. In contrast to the present invention described herein, neither roller is mounted in a manner capable of being biased by a spring. The structure appears to be limited to placement of the rollers on opposing sides of the drive rod with respective axes in planes which are orthogonal (at ninety degrees) to one another. Significantly, the tapered or concave rollers in U.S. Pat. No. 2,619,346 are each of a structure such that only a small single footprint of each roller is intended to contact the drive rod. The rollers are pre-loaded with what appears to be sufficient force to constrain the rollers to single-point contact with the drive rod. Nevertheless, there is sufficient compliance in the bias to permit displacement of the roller axis laterally of the axis of the drive rod. Furthermore, the mechanism disclosed in U.S. Pat. No. 2,619,346 is not a suitable mechanism for application in satellite antenna operations because of the potential for pitch-angle change of the rollers relative to the drive rod and consequent loss of precision control.
U.S. Pat. No. 3,425,284 discloses a linear actuator employing a split block housing. The split block housing is formed in two sections having a compliant interface between opposing halves of the block. Limited pivotal movement is permitted about an axis through the pliant interface to permit changes in the spacing between halves of the split block. While the structure is not strictly used for pre-loading, there is an effect of pre-loading upon compression of the block. Significantly, this linear actuator includes mechanisms for controlling the extent of compression which happens to be performed hydraulically.
Various other patents have been uncovered showing other arrangements for pre-loading traction drive rollers. Reference is made to U.S. Pat. Nos. 3,977,258, 4,131,028 and 4,236,415.
U.S. Pat. No. 4,317,382 describes a drive mechanism in which a sealed housing is employed. Pre-loading is provided by linearly compressing the structures housed within the housing along the axis of the drive rod.
Finally, U.S. Pat. No. 4,451,191 discloses a roller assembly in FIGS. 4 and 5 which suggests the possibility of providing a sealed housing with externally biased compression rollers engaging a drive rod. In this mechanism, however, the rollers are mounted for free pivot with respect to the housing enclosure. It is noted that the rollers disclosed with respect to FIGS. 4 and 5 are specifically indicated as provided with concave or convex surfaces for facilitating engagement with the peripheral surface of the rod. However, the structure as shown would seem to suggest that the alignment of the drive rod is highly dependent upon the relative amount of loading placed upon the drive rod by the three rollers. In other words, no one or two of the rollers are so oriented to fix the position of the drive rod with respect to its housing.